In underwater photography, a water-proof housing is used to protect the camera from water ingress as well as to allow the user to access important functions of the camera from outside the housing. The external controls of the housing are normally in the form of push buttons and levers; the linkages between these controls and the camera are usually mechanical.
The shutter release button of a camera is a two stage button. In the default settings, the autofocus function is triggered by pressing the shutter release button halfway and a picture is taken by pressing the shutter release button all the way down. It is important that the mechanism which links the external control of the housing to the shutter release button of the camera is capable of providing high tactility in order that the user can feel the half press point of the camera button when the camera is placed inside the housing for underwater usage to prevent taking a picture accidentally when the user's intention is to focus. With compact cameras the shutter release control of the housing is usually in the form of a push button.
Digital single lens reflex camera (DSLR) housings are larger in size and rather than the user holding the housing itself, the cameras are often designed to be used with handles. Therefore, the shutter release control on these housings is usually in the form of a lever located on the side of the housings instead of the top so that users can reach the control easily with the index finger without needing to remove the right hand from the handle. O-rings are provided for preventing the housing from water ingress. An E-clip is provided as an aid for the positioning of the mechanism in the housing while a plastic cap on the end of the bar is the contact to the camera.
When the shutter release lever is pressed, a shaft is rotated, leading to the movement of the bar carrying the plastic contact and thereby the plastic contact in the same direction (rotation with axis of rotation same as the shaft), pushing the shutter release button of the camera to trigger the autofocus function and/or taking a picture. A torsion spring is configured so that the lever always returns to the original position after being pressed. Torque is the tendency of a force to cause or change rotational motion of a body. Torque is calculated by multiplying force (the perpendicular component of the force) and distance from the axis of rotation to the point of force application.
When the shutter release lever is pressed, the force exerted Fa (the force component perpendicular to Da, directed outward from the page) produces a torque on the shaft of magnitude:τ=Fα×DαWhere Da is the distance from the axis of rotation of the shaft to the point where the lever is pressed, assuming this point is near the end of the lever (approximately the length of the lever). This torque induces a force Fb on the plastic contact of which the magnitude is dependent on the distance Db, the perpendicular distance between the axis of rotation of the shaft and axis of the plastic contact:τ=Fa ×Da =Fb×Db 
The tactility, in other words, the ability of the user to sense the half press point of the camera is dependent on the ratio of the force induced on the plastic contact, which is also the force the plastic contact comes into contact with the shutter release button of the camera Fb (the output force) to the force exerted Fa (the input force):
            F      b              F      a        =            D      a              D      b      
The internal mechanism of the shutter release button of the camera is designed so that when applying any force slightly additional to that required to reach the half press point, the shutter release will be activated and a picture will be taken. Therefore, the higher the magnitude of Fb/Fa, the more difficult it is for the user the sense the half press point since by only applying a small force, the output force is large enough to overcome the half press point and activate the shutter release. Da/Db can be reduced to increase the tactility, this can either be done by decreasing Da or increasing Db, however, there are limits to the amount of adjustment of both lengths. Da must be large enough so that the user's index finger can fit comfortably when pressing the lever; besides, changing Da affects the stroke length, which is an arc of which the length is proportional to the radius of circle i.e. Da and it is unfavourable to reduce Da such that the stroke length becomes too short.
Increasing Db is also difficult as the space between the camera and the housing is very limited because smaller and lighter housings are often preferred.